So I have always had this problem as well.
But really after lots of drawing and lots of painting you realise that its just time, and marks, curved or straight, and edges soft or hard, and contrast.
But there are a few things you can go by for yourself for now if a piece is finished or not.
ONE: Ask yourself questions like your audience, or art director.
The alien is humanoid enough for me to understand her emotions. But I have questions. And you don't want me, the viewer, to have any questions at all. You have to sell sell sell me the story. "What is that....blood coming off the hands?", "Is she mad or stoic or just annoyed...she looks annoyed?", "Is it the blood on the hands making her annoyed or how goofy the guy is?", "Is her head dress floating in the back or is the artist finished?", "Is that guy homeless or just happy to see someone after a long space flight?"
That being said my advice is to think like the viewer and ask yourself these questions and answer them by doing the painting or drawing.
TWO: Face and hands, face and hands, face and hands, face and hands, face hands facnedshandandsfands face hacefancds face and hands. You get it.
After shape language and composition we identify the human emotions by these two anatomy parts the most. Nail them down every time. @cedricgo is right about the hands. Sell sell sell me that this guy is overly excited. Don't know what to do next in your piece? Well just go ahead and knock these two parts off the list while you come up with some questions. They take the longest every time for me because of so much identifying information.
-- Here's a scenario that's never happened that illustrates my frustration with hand painting. -
Final is presented.
Art director: "Could you maybe move his hand onto his waist? "
Me: Breaks stylus. 
"Welcome to my *day rate."
Art Director: "Nevermind its fine."
Three: Take a breath. And a break and come back and just realize that all you have now are edges. Because at this stage of your paitning all of your hard thinking is out of the way here. At this stage in your painting you should be done with Color/light block in, pose, design, and composition. And the process that I enjoy the most, painting and rendering, can now begin at this point while I listen to a book or podcast or watch a streamer and just spend time rendering. Edges of contour, light, and surface texture. Hard vs. Softer.
And that's really it. If I did a paintover it would just be the finishing stage of rendering form better but I'll leave that to you.
Four: Will it look good if I print this?
I like to print my stuff and it never looks the same in rgb on screen as it does on paper and I adjust my painting process to do so. I would need to see more of your stuff to tell for sure but I can tell that your screen is bright and the purple part of her arm in shadow (on the left) is not going to print well and is way darker than the other arm which also has shadow.
Keep all your values blocked in the same range for easy read and identification.
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And that's about it! I hope that helped you think about finishing your piece and know that its not that big of a deal and not to get too overwhelmed. You only get overwhelmed if you dont know whats next. If you finish it please post it over in finished pieces and update us here on this thread with a link so we can see. Best of luck!
